350 Chevy Bolt-On Power Mods

Dyno Testing
We brought our stock 350 to Westech Performance Group's dyno facility in its baseline stock configuration, complete with an iron intake manifold and Q-Jet carb. To facilitate dyno hook-up, the engine was configured with a set of Westech 1 3/4-In. sprint-car style dyno headers, which are equipped with the required fittings for EGT connections and a wide band Lambda sensor. Unfolding our plans began with running the engine for a baseline, which would give a chance to validate the stock power output. Before long, the engine was running on the SuperFlow dyno, and up to operational temperature. Our initial pulls showed a disappointing 214 hp and 307 lb-ft. This was a good 76 hp short of the 290 hp the crate engine is rated for, so what happened? It didn't take long to spot the anomaly on the dyno sheet-- it had recorded the intake manifold vacuum reaching a peak of 8.8 in-hg at 5,300 rpm!

There is only one explanation for that, the secondaries in the Quadrajet were not opening. We re-adjusted the tension at the secondary air valve, setting it loose, and made certain that the choke lock-out linkage was clear, and finally got our valid baseline, 299 hp at 5,000 rpm, and 350 lb-ft at 3,400 rpm. Considering the difference between the SAE correction factor used by OEM manufactures (from which this engine's ratings were derived), and the STP correction factor used by the performance aftermarket, the numbers were now right in the expected zone.

Our first mod was to unbolt the heavy iron lung from between the heads, and bolt in the sweet Edelbrock AirGap intake. To top the manifold, we pulled out a Mighty Demon 750 cfm carb. The Mighty Demon line is aimed at serious street performance, but is packed with most of the features of Demons Race Series carbs--mechanical secondaries, replaceable air bleeds, a contoured air horn, and no choke. Being from mild Southern California, we never miss the choke, but we certainly appreciate the power delivered by these carbs. We were nothing short of impressed with the results of the induction change, with the dyno now registering peaks of 348 hp and 370 lb-ft of torque, at 5,300 and 4,200 rpm, respectively. Frankly, the increase was about the biggest gain we've seen in a carb and intake swap, proving once again the effectiveness of this outstanding combination. Torque was up by 20 lb-ft, while power was boosted an incredible 49 hp. This is definitely hardware worthy of consideration for any high performance effort.

With the induction system tested, we were left with the RHS cylinder heads and COMP hydraulic roller cam. Our inclination was to move to the cam swap first, and test the effect of a cam change alone, however this proved to be problematic. We knew that the valvesprings would need to be upgraded in order to run the hotter cam. We found that the exhaust retainers would also need to be changed, since the engine was equipped with exhaust valve rotator style retainers, which reduced the installed height to the point of making coil bind an issue. The clincher was finding insufficient retainer-to-guide clearance to accommodate the cam's lift. It would take removing the heads, and machining the guides to get the job done. All in all, it would take a significant amount of reworking to get the stock iron heads set up to handle our cam choice, including upgrading the studs, and machining the spring seats.

While these hurdles can certainly be crossed, we decided the best course of action was to consider the cam and head swap as a single package. After all, the full benefit of the cam would not be realized without cylinder heads that flow well at higher lifts. On the flip side, it seemed like an unlikely combination to bolt on aftermarket heads with the small factory cam. The cam just wouldn't make use of the heads, and negate much of the available power potential. Either the cam or the heads alone would be a questionable combination, and unlikely in the real world. What works is a matched cam and cylinder head combination that complement each other, and these two parts were originally selected to work well together.

We opened the 350 up for the cam and head swap, completing the installation with the COMP Pro Magnum 1.6:1 rockers. We found the performance parts were easy to install, replacing the original pieces perfectly. The retrofit hydraulic roller arrangement replaces the stock flat tappets with paired lifters joined with link bars to establish the roller alignment, unlike the OEM hydraulic roller arrangement that employs alignment yokes and a spider assembly. With the RHS heads, the 350 took on a much more purposeful performance look, but the bottom line would be the power output. We were pleased to find the idle quality was more than acceptable for a street engine, with a mild lope at an 850 rpm idle, and about 13-14 in-hg of manifold vacuum. The smaller combustion chambers of the 64cc RHS heads added a solid point of compression ratio, which is needed compensation for the longer valve events of the COMP cam. We found 165 psi of cranking compression with the new combination, low enough to be happy with even lower grade pump gas, but high enough for decent low-end torque and power.We were shooting for a solid 400 hp from this basic and street-friendly 350. The dyno would soon tell us if the combination would be enough to reach our goal.

The willing small-block easily covered that mark, posting a peak of 412 hp at 5,600 rpm, with torque now reaching 421 lb-ft at 4,100 rpm. These are good numbers for a real street 350 based on a typical production-style low-compression bottom end. The cam and head swap was worth a solid 64 hp gain in horsepower, an increase of over 18 percent. Torque was up a substantial 42 lb-ft, and the torque was up everywhere in our test rpm range by about that amount. It is unusual to find big torque gains across the board, through a broad rpm range. The engine was clearly much happier with the new combination.

We felt that the engine's output struck a very good balance of useable power for a street driver that operates in a moderate rpm range and would run happily on low-grade fuel. For some added sizzle, we figured the engine could reliably handle a 100 hp hit of nitrous. We like the idea--412 trouble-free and reliable horsepower, packing an extra "C-note" of power when the occasion calls. We tapped ZEX for one of their jettable perimeter plate nitrous units, wired up the solenoids, and routed the trigger switch to the dyno control center. Just for additional safety, we pulled 6 degrees of timing, dropping the total timing from 36 degrees to 30 degrees, which reduced the normally aspirated power by about 10 hp. Ingesting nitrous, the results were as predictable as they were satisfying. We hit the squeeze at 4,200 rpm, and power spiked with a huge torque surge to peak at 584 lb-ft at 4,500 rpm. Horsepower cranked up to nail the 500hp mark, and just hovered at that power level all the way to 5,700 rpm, where we let go of the nitrous trigger. This engine seemed to just love the modest shot of nitrous.

With a few well-chosen bolt-ons, our "Plain-Jane" 350 gained 112 hp in normally aspirated form, and over 200 hp with a livable little shot of juice. Even better, it gained that power without killing the drivability and usefulness of the engine in a real street application. On the motor, it made enough power not to have to make excuses. With a modest shot of nitrous, we could shove the power upward to where it would match power levels normally seen with very radical and raucous small-blocks. It's a nice combination to have, without the drawback of living with and feeding a temperamental rascal on a daily basis.